The present invention relates to powering high intensity discharge (HID) lamps, and more particularly, to a novel circuit for continually monitoring and adjusting lamp current and voltage.
First automobiles of the late 1800s were not equipped with headlights. Automobile headlights were added in 1885 to allow travel in the evening hours. The first electric headlamps appeared in 1905 and eventually became the norm. Sealed beam headlights were used into the 1970s and replaced by halogen headlamps in the 1980s and '90s. At present, the halogen headlights are being replaced by HID (High Intensity Discharge) lamps.
The HID lamp provides a high-intensity light produced from an electric arc inside a capsule filled with xenon gas. The arc produces significantly more light than a glowing filament found in the halogen lamps. The HID lamps draw less power from a vehicle's electrical system, they are more durable, have higher intensity, longer life, and better directivity than their halogen counterparts.
The HID lamps produce light by striking an electrical arc across tungsten electrodes housed inside a specially designed inner glass tube. This tube is filled with both gas and metals. The gas aids in the starting of the lamps and the metals produce the light once they are heated to a point of evaporation. The HID lamps produce a large quantity of light in a small package.
An HID lamp typically requires a higher voltage, e.g., 400V, for ignition, which is followed by an operating region in which voltage is lower, e.g., 100V, with current in the range of a few amperes. In this operating region, it is desirable to maintain a constant power output. It is also desirable to achieve a constant luminescence, even during the initial startup period, particularly for automotive applications.
It is important for longevity of an HID lamp to regulate the power supply to the lamp during normal running conditions. Accordingly, it is desirable to provide constant power to the lamp during normal operation to maintain an even light output and extend the longevity of the HID lamp.
A typical prior art solution for a HID ballast circuit (FIG. 1) includes a boost stage 10 and a full-bridge inverter stage 12. The boost stage 10 boosts the DC battery voltage and regulates the DC bus output voltage to a typical value of 400VDC. The full-bridge stage 12 drives the lamp 13 at a low frequency (200 Hz typical) and provides the AC voltage waveform across the lamp 13. The boost stage 10 is typically controlled with a control IC 14, of which several are marketed by various IC manufacturers. The full-bridge stage 12 may be controlled using two International Rectifier IR2153 self-oscillating gate driver ICs 18. Discrete control circuitry 19 is typically used to control the full-bridge stage 12 and performs the following functions:                1) Lamp ignition (ON/OFF control of the ignitor circuit 20)        2) Senses lamp voltage and current        3) Detects various lamp fault conditions        4) Provides ignition timing        5) Counts the number of fault events        6) Resets the ballast or turns the ballast off when faults occur or when the fault counter has timed out.        
This solution typically requires a large number of control ICs resulting in a high component count, large area of Printed Circuit Board (PCB) space, high manufacturing costs, and high overall ballast circuit cost. A more elegant solution is desired for integrating as many functions as possible into a single IC to reduce component count, reduce PCB board space, reduce manufacturing costs, reduce overall ballast cost, and increase reliability. Moreover, there is a need for a constant power control circuit that will deliver and maintain a constant power to the automotive HID lamp and further provide a constant lamp brightness even during the critical warm-up period. Further an HID ballast which provide for a hot restrike of the lamp in the event the lamp goes out is also necessary, particularly in an automotive application.